Wow! Signal

This one's a little more recent, but it is essential for any unsolved mystery list. On August 15, 1977, a SETI (Search for Extraterrestrial Intelligence) worker named Jerry Ehman detected a strong narrowband radio signal from somewhere in the constellation Sagittarius. It was completely unlike any natural phenomenon ever observed by cosmologists at the time or since, and is widely speculated to have been an artificial transmission from an alien civilization. The E.T. hypothesis is supported by the sharp, clean frequency of the signal, and the fact that it was 1420 MHz. That's right on the hydrogen line, a universal standard so distinct that even we figured out it should be reserved for radio astronomy. Unfortunately, nothing like it has ever been detected since, so the mystery of the aptly named Wow! Signal remains unsolved for now.